One or more embodiments relate to a method of operating a nonvolatile memory device.
Recently, there has been an increasing demand for nonvolatile memory devices which can be electrically programmed and erased and do not require the refresh function of rewriting data at specific intervals.
A nonvolatile memory cell enables an electrical program/erase operation and performs the program and erase operations by varying threshold voltages when electrons are migrated by a strong electric field applied to a thin oxide layer.
A nonvolatile memory device may include a memory cell array in which cells configured to store data are arranged in matrix form and a page buffer configured to write data into specific cells of the memory cell array or to read data stored in specific cells of the memory cell array. The page buffer includes bit line pairs connected to specific memory cells, registers configured to temporarily store data to be programmed into the memory cell array or to temporarily store data read from specific cells of the memory cell array, a sense node configured to sense the voltage level of a specific bit line or a specific register, and a bit line select unit configured to control whether or not to connect the specific bit line and the sense node.
In the verification operation of such a nonvolatile memory device, there is a concern that read margin decreases since under-programmed cells are obtained due to a source line bouncing phenomenon. Further, a bouncing phenomenon in which the voltage level of a source line varies depending on the program state of neighboring cells (for example, cells at an end) occurs. Accordingly, the level of current passing through a specific cell is changed. Consequently, cells, which are determined to have been programmed, while actually not being fully programmed (i.e., under-programmed cells), are obtained.
Cells, which have not been programmed to have a first reference voltage PV1 or more, may be determined to have been programmed. Accordingly, read margin, corresponding to the difference between the first reference voltage PV1 and a read voltage Vread, decreases. This phenomenon may be detrimental to a Multi-Level Cell (MLC) program method. This is because, in the MLC program method, a read margin between two or more distributions of threshold voltages is desired to be large.